The motivation of frame rate up-conversion is that a video sequence with higher frame rate is generally considered to give higher quality experience than a video sequence with lower frame rate. The frame rate of a video sequence can be increased by inserting predicted frames in between existing frames. A good approach is to predict the in-between frame using bi-directional block based motion estimation [1], searching for linear motions between the previous frame and the next frame in the input video sequence. It is possible to use non-linear approaches that can represent acceleration, but the linear approach is used because of its simplicity and low complexity. The in-between frame is divided into blocks and to each of these a motion vector must be assigned in some way. However, it is possible that a set of motion vectors are candidates for the current block, and only one of these candidate motion vectors will be selected to represent the motion.
When an evaluation is performed concerning which of these candidate vectors to choose, there are different parameters that can be used. The most common is the sum of absolute differences (SAD) [2], which is calculated from the differences of the values between the pixels pointed out in the previous frame and the pixels pointed out in the next frame.
In some cases two (or more) different motion vectors will have low SAD value but only one can be selected to represent the motion. FIG. 8 shows two frames 10, 20 in a video sequence. It is not hard (for a human) to guess the contents of the in-between frame 30.
Since the background is unchanged it would probably be the same as in the two adjacent frames 10, 20, and since there is a ball to the left in the preceding frame 20 and a ball to the right in the following frame 10, it is intuitional to think that there is a ball in the middle of the in-between frame 30. The motion vector that points from the ball in frame 10 to the ball in frame 20 gives a low SAD value and passes the middle of the image at half of its length.
However, the area in the middle looks exactly, or almost exactly, the same in the preceding frame 20 as in the following frame 10. A local consideration would yield that this area will look the same in the in-between frame 30, and the zero motion vector gives a very low SAD value. Thus, the prior art techniques for selection motion vector may run into problems in this situation. Since the zero motion of the background is the easiest to capture exactly, this motion vector will be chosen instead of the motion vector that corresponds to the motion of the ball. This could result in an in-between picture 30 with no ball at all.